BL2 2 players splitscreen FPS (PS4)

Hi, i am going to be as clear as possible as english is not my native language. Me and my sister started playing yesterday Borderlands 2 as we loved the first one, but we are noticing some frame rate drops when we are moving around the scenario, in big areas (not very noticeable), when in the middle of a gunfight, the frames starts dropping, not that much, but it’s a little annoying. The worst is when one of us open the inventory / menu, the games becomes so much slow. And we are only 2 persons, not even 4. And i heard the pre sequel has some troubles even when solo. Is there some patch in progress for the splitscreen drops? (BL2 i am talking) why does it lag when someone opens their menu/inventory?. Regards